- One of the most commonplace microbes, Epstein-Barr virus,
may play a key role in triggering the debilitating neurological disease
multiple sclerosis, Canadian researchers have discovered.The link between
EBV, the virus that can cause mononucleosis, and MS has long been suspected
but research conducted at the Toronto Hospital for Sick Children provides
some of the strongest evidence yet that childhood exposure could be an
important factor.
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- "Our thinking is that it's not whether you get the
virus that's important, but when," Dr. Brenda Banwell, director of
the hospital's pediatric MS clinic, said in an interview. "We think
it may be a timing and vulnerability issue."
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- The research, published in today's edition of the Journal
of the American Medical Association, was conducted by looking at blood
samples from children with MS, children who came to the emergency room
and healthy children in the community.
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- About 83 per cent of the children with MS had been exposed
to EBV, compared with 42 per cent of the other children. Exposure rates
to a number of other common viruses were similar in the two groups, with
the exception of herpes simplex virus (the virus that causes cold sores).
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- Children with MS were actually far less likely to have
been exposed to herpes. This led researchers to think that the order in
which children are exposed to common microbes may also play a role in their
risk of developing MS. In other words, an immune system that has already
wrestled with herpes, may be better prepared to deal with EBV.
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- Multiple sclerosis is a bedevilling disease of the brain,
spinal cord and optic nerves that can cause problems with muscle control
and strength, balance, vision and sensation.
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- More than 50,000 Canadians have MS. In fact, Canada has
one of the highest rates of MS in the world.
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- In people with MS, for reasons that are unclear, the
body's immune system malfunctions and starts attacking myelin, the protein
coating that surrounds and protects nerve fibres.
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- This is an important link because the genetic code of
the Epstein-Barr virus contains sequences that are identical to genetic
sequences in myelin protein.
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- Dr. Banwell said it is "conceivable that the immune
system mounts a response to that genetic sequence in EBV, then sees the
myelin and targets it as well." In other words, the immune system
thinks it is attacking the virus when it is actually attacking healthy
nerve cells.
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- With this information, scientists could, theoretically,
find a way to stimulate the immune system so that it does not attack myelin
protein and hence prevent some cases of MS.
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- Dr. Banwell stressed, however, that multiple sclerosis
is a complex autoimmune disease, and that there are no doubt a number of
triggers.
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- For example, there is a lot of research that links MS
to lack of exposure to sunlight. That could explain why countries farthest
from the equator, such as Canada and Australia, have the highest rates
of MS.
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- That research suggests that lack of exposure to ultraviolet
light, or to vitamin D, may be a trigger. Recent research has shown a certain
type of cell in the immune system, called a T helper cell type 1, leads
the attack on myelin, and that exposing the T helper cells to ultraviolet
light can stop those attacks (at least in laboratory mice).
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- But another possibility is that people in northern countries,
because of their extreme weather, spend more time indoors, and are more
likely to be exposed to common viruses such as EBV and herpes simplex.
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